[This 5-year grant proposal is a competitive renewal of the currently active grant entitled "Crystal structure of Ion Channels" funded from 3/1/97 through 2/28/01.] This renewal approach entitled "Tertiary Structures of Potassium Channel Domains", is a continuation of our efforts on molecular understanding of mechanisms of potassium channels for ion conduction, voltage-activated gating, and channel regulation through interaction with cytoplasmic component. Two main experimental goals have been designed to understand the mechanisms of (1) assembly of potassium channel subunits into a functional tetramer and (2) its interaction with the cytoplasmic regulatory protein. To reach these goals, we propose the following x-ray crystallography experiments as the backbone of the proposal. 1) We will determine the three-dimensional structure of the entire N- terminal tetramerization domain of a potassium channel. 2) We will determine the three-dimensional structure of a regulatory protein for a potassium transport in prokaryote. As the structural analyses of these two domains progress, we will follow up with molecular characterization of these domains using site-specific mutagenesis and other biophysical tools to further understand their functional roles. Deciphering the structural basis for channel assembly through tetramerization is essential to understand the physical nature of channel diversity. From the atomic model of the regulatory domain, we will seek to establish a knowledge basis that will be generally applicable to a variety of ion channels to decouple their channel actions with the biochemistry of the cell. Detailed atomic model will undoubtedly be useful as a template for structure-guided drug design towards various neurological disorders involving potassium channels.